Ultraviolet Line Spectra of Metal-poor Star-forming Galaxies

We present synthetic ultraviolet spectra of metal-poor star-forming galaxies that were calculated with the Starburst99 package. A new spectral library was generated from Hubble Space Telescope observations of O stars in the Large and Small Magellanic Clouds. The corresponding mean metallicity of the synthetic spectra is approximately 1/4 Z_solar. The spectra have a resolution of 1 Å and cover the spectral range 1200-1600 Å. A set of model spectra was calculated for a standard initial mass function and star formation history and is compared to synthetic spectra at solar metallicity. We find that the spectral lines are generally weaker at lower metallicity, as expected from the lower elemental abundances. Stellar wind lines, however, show a more complex behavior: the metallicity dependence of the ionization balance can be important in trace ions, like N⁴⁺ and Si³⁺. Therefore, the strength of N V λ1240 and Si IV λ1400 does not scale monotonically with metallicity. We compare our new models to ultraviolet spectra of NGC 5253 and MS 1512-cB58, two star-forming galaxies with one-fourth solar metallicity at low and high redshifts, respectively. The new library provides significantly better fits to the observations than earlier models using the Z_solar library. We discuss the potential of utilizing stellar photospheric and wind lines to estimate the chemical composition of star-forming galaxies. The new metal-poor synthetic spectra are available via the Starburst99 Web site.